Example Task Question : The channel characteristics of a river change along its course.

Transcription

1 Example Task Question : The channel characteristics of a river change along its course. Purpose Focus on velocity Sub focus questions: 1. To what extent is there a regular /even change in velocity downstream? 2. Do changes in velocity downstream match the expectations of the Bradshaw model? 3. What are the reasons for the change in velocity with progression downstream? Location and background research The River Horner is located in the southwest of England in Somerset. It is found on the northern edge of Exmoor (Figure 2) and has its source about 40km north west of Taunton. In its lower course the river runs through Horner Wood, between Porlock and Allerford and through the village of Bossington. The river tends to flow in northerly direction from source to mouth and is 9km long. This makes it ideal for study since a number of sites along the whole river can be accessed relatively easily. The main valleys between the hills are filled with alluvial deposits from the hills or sea. Figure 2 is a geology map that was sourced from the British Geological society (GIS kml Google Earth overlay ). Figure 1 opposite Bradshaw s model (sourced from the internet) Brashaw s model will provide the focus and context for this study. Figure 2 Geology map of south west England. A risk assessment is also included as part of this purpose. Note how risks have been managed..

2 What are river characteristics? We concluded that they could include: width, depth, velocity, gradient, sediment size etc. Velocity works as an important control factor in all of this. Figure 3a and 3b (above) location of the River Horner (Source OS and ), plus inset regional map. Scale 1km = 1 square. Note river is indicated in blue on the right hand map. Risk Assessment Hazard Risk Likelihood Control Traffic Hit by moving vehicle Low Careful of vehicles, cross at designated points; awareness Rocky footpath and Uneven ground Ticks Slipping and trips, scratches and cuts, twisting ankle Limes disease and / or a bite High Medium Awareness and care, move slowly and watch out for each other Long sleeves and awareness Planning and methodology A collaborative exercise was used to think about the influences on river velocity (thought to be very important in terms of characteristics) and then a mind map was created online ( ). This helped me to prepare for the fieldwork.

3 It was decided that velocity would be measured using a float method. This was the most practical approach give the limitations of equipment. The float method is based on speed = distance / time (see opposite source Choosing a float is also important here. After consultation with a number of textbooks, it was decide that dog biscuits would be used. These float just below the surface and are therefore not affected by the wind. We timed them over a distance of 5 metres. This was repeated 5 times at each site to get a more reliable average. Figure 4 opposite (photograph) shows accurate recording of times. Ten sites were selected down the River Horner. This was considered to be enough to link to the Bradhaw s model so that significant differences could be seen. The sites are shown on Figure 4. Other variables were measured by other groups; this information may be used later as part of the understanding. Systematic sampling was used as it is straightforward to carry out, but in some places the systematic approach had to be abandoned due to access difficulties so the sites were not always evenly spaced..

4 Figure 3 GIS Google Earth map showing the location of the River Horner, plus fieldwork sites, with an OS grid overlay (source ). Systemic sampling was used when there is an expected change between two locations (an 'environmental gradient'), then systematic is used to take regular samples at known distances. Therefore this is suitable for the River Horner.

5 Methods of presenting data A scatter graph is the most appropriate way of indicating the strength of relationship between site number and distance downstream. Figure 4 shows this, including a line of best fit which has been added using Excel. Photos also show changes in the river channel characteristics from the upper to the lower sites. Figure 4 our groups data Velocity m/s Velocity Linear (Velocity) Site number The model indicates that velocity should increase with distance downstream Data from the other groups has also been used, but this forms part of the analysis.

6 Figure 5 Flow diagram shows changes in velocity along the course of the River Horner. Length of arrow is proportional to average group velocity (total sample) Key 1cm = 0.1 m/s Using other groups data (pooled set) we were also able to plot a cross section based on one groups data for site 7 (Figure 6).

7 Secondary data from the National Rivers Flow Archive allowed us to find out more about the catchment of Exmoor. The reason is that discharge is proportional to discharge. This will help with the evaluation of the data. See Figure 7 (spreadsheet data from NRFA plotted for 2011 to create a regime) Discharge maximum for day (cumecs) Exmoor Catchment Data Data analysis and conclusions The data shows that the velocity of the stream starts off quite slowly at 0.16m/s at site 1. It then rapidly quickened up to 0.41 and 0.38 m/s at sites It then dropped again in the middle sites (0.05 and 0.08 m/s) before gaining speed again at site and 0.31 m/s. Overall this shows a very mixed pattern for velocity with considerable range ( = 0.46 m/s). The average velocity is 0.24 m/s, but the median is slightly higher at 0.25 m/s. This is not really a very good measure of central tendency since the data seems to be set into two groups: Group 1 = low values ( ) Group 2 = higher values ( ) Site 1, Site 4, Site 5, Site 6 Site 2, site 3, Site 7, Site 8 I also used an Excel spreadsheet to look for any correlations between the site and velocity see screen print for evidence (Table 1). My hypothesis was: There is a relationship between velocity and distance downstream (Null = There is no relationship between velocity and distance downstream)

8 This correlation figure was calculated using Excel. A very low number of 0.13 means that the results are not statistically significant. Therefore the null hypothesis should be accepted, i.e. there is no linkage between distance downstream and stream velocity. This is also verified by the scatter graph, Figure 4. A further part of the analysis involved looking at other groups data to see whether there were any more obvious patterns that were different to our own, individual groups data. Table 2 group data with quartiles (Q1 + Q4) and median and standard deviation. Q1 Median Q4 SD Site The results are most reliable at sites (lowest standard deviation), whereas sites have greatest variation. I have also calculated the upper and lower quartiles Q1 (Lower) + Q4 (Upper), plus the median values for all the groups at each site. Figure 8 plot of quartiles, median and standard deviation for all groups data on the River Horner. This shows variations both downstream and between groups; perhaps median is the most reliable indicator of change.

9 Velocity m/s Group data for velocity Site number Lower Quartlie Median Upper Quartile SD The graphs also back up the idea that velocity does not change evenly with distance downstream. This doesn t fit so well to the original model, at least for velocity. The model is idealised and simplified. Conclusions and evaluations Overall there is only limited linkage between the results obtained on the River Horner It is certainly obvious that there are some unexpected readings. According to geographical theory, velocity may be linked to some other variables see my model below (Figure 7): Discharge of the stream (velocity is a component of this) Cross sectional area at point of measurement Likely influences on the River Horner velocity Gradient of the river Size of stones (influencing the friction) Note the thickness of the arrows indicate how strong I think the relationship is between the different factors. I think t the gradient of the river is least important factors as this doesn t really change much in the River Horner, although it is steepest at Site 1. It is very gentle from site 4 to site 8.

10 Although there were a number of anomalies in my investigation of the velocity of the river Horner, generally velocity gradually increased as the distance downstream increased: 1. There is a greater volume of water in the river as it goes down stream due to tributary rivers, as a result of this the momentum is greater and there is a greater velocity. 2. The river channel becomes more efficient with distance from the source, due to meanders and the gradual change of the river to equilibrium the velocity accordingly increases. 3. Finally, the velocity increases because of particle sizes, which decreases. More energy can then be put into the movement of the water in the channel rather than the movement of particles. Returning to the main focus questions, I have provided my responses using descriptors in blue next to each one: 1. Do changes in velocity downstream match the expectations of the Bradshaw model? Only partially. Results are inconclusive based on either my group s data, or other groups. Bradshaw s model is catchment based, whereas we just look at small section 2. To what extent is there a regular /even change in velocity downstream? This is not the case. The data suggests two groups. 3. What are the reasons for the change in velocity with progression downstream? These can only be guessed at, but must include friction, gradient, channel roughness and possibly land use. Antecedent conditions may also have an impact. As could local changes in geology. A repeat study of the same area and under different conditions may help answer these questions. Evaluation There were a number of limitations to our study, particularly that we only had time to visit each site once and we could not repeat the whole experiment. We experienced a number of human limitations as well in that we had to try and measure speed accurately with the float. If we repeated the study I would suggest that we decide on a larger number of fixed number of points to investigate across the river, so that we get a more accurate set of results. Using the method that we used, we investigated a different number of points, at intervals at each site, which did not provide the most accurate results, as shown by the number of anomalies. Data presentation was limited in some respects, e.g. only showing one cross section and also assuming in Figures 4+7 plotting the sites equidistant from each other (they were not). Secondary data was for the whole catchment rather than just this area, so once again it may not be fair to compare our results to a larger scale drainage basin. There are other pieces of data that could have been collected that would have taken the investigation further.

11 1. Larger number of sites; more measurements at each site would improve reliability. 2. More in depth analysis of channel roughness (hydraulic radius) as this is likely to affect velocity. This could have been measured at the same sites also. 3. If gradient is control over velocity then this should be measured. I could get this from a GIS map, as well as other group s data. 4. Greater use of secondary data, e.g. other past projects, or monitoring stations. 5. More photographs would have provided greater evidence. To really improve the work, information could have been recorded from different times of year under different river flow (regime) conditions. Changes in velocity downstream may only be apparent, under higher flow conditions, for instance. This is when most sediment is moved and erosion takes place. Our results are a snapshot survey so we should question their validity, especially in the context of Figure 7. Overall channel characteristics do change along its course, but the relationship is far from simply predictable or easy to understand.

12 Examiner comments purpose of investigation (max 6 marks) Mark L3. 6 marks. A very clear set of focused questions relating to the aims and task set. Study clearly located with some use of different types of GIS / digital maps clearly linked to overall task question provided by Edexcel. Evidence of research in the work also linked to question and secondary data obvious linkage to models and theories; a succinct risk assessment is provided Well written and tightly focused. Shame there is no key on the geology map. Maps may have been re ordered so that the site map came first. They should also all include scales. River could be more clearly located. However this should be rewarded at the top of L3. Examiner comments methods of collecting data (max 9 marks) Mark L3. 9 marks. A good description of the methods plus their location on a GIS map from Google Earth which is good practice; some originality. Evidence of a range of collaborative work which should be encouraged. In all instances there is also justification. The methods are clear, logical and appropriate for an investigation of velocity and it is apparent that this student has made a real contribution to the process. There is also reference to sampling. A risk assessment is provided as part of the introduction, but of course is relevant to this section. Secondary data should be credited from research, e.g. finding out about methods as well as data sets. This student has done both the later in the form of a river regime which comes from the NRFA. Possibly the work may have been improved with annotated examples of recording sheets, however it still deserves top marks. Examiner comments methods of presenting data (max 11) Mark L3. 11 marks. A good range of range of presentation techniques, including some original ideas using ICT. All the graphs and data are relevant to the question and well selected. There is good evidence of complexity and sophistication, e.g. Figures 4, 5 and 6. Figure 7, based on secondary data from the National Rivers Flow Archive is accurately constructed from a spreadsheet. Perhaps a missed opportunity for using GIS to present some data; also there are no photos or field sketches included. But as the work has a focus on velocity rather than landforms, this may be the reason. Once again this section should be rewarded maximum marks. Examiner comments analysis and conclusions (max 9) Mark L3 9 marks

13 The analysis and conclusions are written in a very clear and coherent style. The integration of diagrams is good practice and improves readability (this can be done under high level of control as long as no new material is introduced). There is lots of evidence of analytical thinking easily enough for top band, e.g. use of modes, means medians etc. Clear reference to Geographical theory. Most important here is that it is possible to see understanding in this section. The students clearly gets it and can draw on their own knowledge and understanding of the topic to present very sensible ideas. Of particular note is the clever way in which they have split the river into two distinct sections based on its velocity characteristics this is evidenced in Table 1. Maximum marks should be awarded for this section. Examiner comments evaluation (max 9) Mark L3. 9 marks In many ways the evaluation of this work is its most sophisticated element Not only is there a very through end evaluation which identifies problems and suggests sensible workrounds, but there is also much on going evaluation present. It is perhaps the last paragraph, when the students refers to Figure 7 that really demonstrates high order thinking skills. Overall comment: An excellent study which is very tightly focused just on velocity which has made it manageable in terms of scale and length. It is highly geographical and linked to an appropriate model which facilitates clear understanding and a reflective style. The work is very clearly structured and well written using appropriate geographical terminology. Word length is about right a fraction over The best parts of this work are arguably the sections done under high control. The analysis is very sound, and it was a good idea to compare individual group s data with some of the other groups. The statistical analysis is very good for GCSE work and has given the student something meaningful to write about. The fact that the work is wholly focused on velocity (a contextualisation of the original task) has ultimately made this very successful indeed.